Math Problem Statement

A uniform thin rod of mass 0.950 kg and length 1.20 m is mounted on a horizontal pin through the center of the rod at right angles to the rod such that the rod is free to spin in a vertical plane about the pin without friction. The rod is positioned horizontally at rest by a person who then releases the rod. The rod remains at rest until a ball of mass 0.265 kg is dropped on the rod from a height of above theβ„Ž0 = 1. 400 π‘š rod. The ball hits the rod at a point exactly between the center of the rod and one end of the rod. The ball bounces straight up to a maximum height of 0.599 m above the point of contact. How fast is the rod spinning after the ball hits the rod the first time but before the ball hits the rod again? (Note that the moment of inertia of a uniform thin rod, with respect to an axis that passes through the midpoint of the rod and is perpendicular to the rod, is given by where M is the mass of the rod and L is the length of the rod.) 1 12 𝑀𝐿2

Solution

To solve this, we use the principles of conservation of angular momentum and energy considerations.


Step 1: Information and Formulas

Given:

  1. Rod mass (MM): 0.950 kg0.950 \, \text{kg}
  2. Rod length (LL): 1.20 m1.20 \, \text{m}
  3. Ball mass (mm): 0.265 kg0.265 \, \text{kg}
  4. Height from which the ball is dropped (h0h_0): 1.400 m1.400 \, \text{m}
  5. Height the ball reaches after collision (h1h_1): 0.599 m0.599 \, \text{m}
  6. Moment of inertia of the rod about its center: Irod=112ML2I_\text{rod} = \frac{1}{12} M L^2

To Find:

Angular velocity (Ο‰\omega) of the rod after the collision.


Step 2: Linear Momentum of the Ball Before and After Collision

The velocity of the ball before impact is calculated using: vbefore=2gh0v_\text{before} = \sqrt{2 g h_0} The velocity of the ball after impact is calculated using: vafter=2gh1v_\text{after} = \sqrt{2 g h_1} Where g=9.8 m/s2g = 9.8 \, \text{m/s}^2.


Step 3: Angular Momentum Conservation

The system's angular momentum about the pivot is conserved. Initially, only the ball has angular momentum since the rod is stationary.

Initial Angular Momentum (Before Collision):

Linitial=mvbeforeβ‹…L4L_\text{initial} = m v_\text{before} \cdot \frac{L}{4} where L4\frac{L}{4} is the perpendicular distance from the pivot to the point of impact.

Final Angular Momentum (After Collision):

After the collision, the rod rotates with angular velocity Ο‰\omega, and the ball continues with linear velocity vafterv_\text{after}. The total angular momentum is: Lfinal=Irodβ‹…Ο‰+mvafterβ‹…L4L_\text{final} = I_\text{rod} \cdot \omega + m v_\text{after} \cdot \frac{L}{4}


Step 4: Conservation Equation

Equating the initial and final angular momenta: mvbeforeβ‹…L4=Irodβ‹…Ο‰+mvafterβ‹…L4m v_\text{before} \cdot \frac{L}{4} = I_\text{rod} \cdot \omega + m v_\text{after} \cdot \frac{L}{4} Substitute Irod=112ML2I_\text{rod} = \frac{1}{12} M L^2 and solve for Ο‰\omega: Ο‰=mvbeforeβ‹…L4βˆ’mvafterβ‹…L4112ML2\omega = \frac{m v_\text{before} \cdot \frac{L}{4} - m v_\text{after} \cdot \frac{L}{4}}{\frac{1}{12} M L^2}


Step 5: Substituting Values

  1. vbefore=2β‹…9.8β‹…1.400=5.242 m/sv_\text{before} = \sqrt{2 \cdot 9.8 \cdot 1.400} = 5.242 \, \text{m/s}
  2. vafter=2β‹…9.8β‹…0.599=3.430 m/sv_\text{after} = \sqrt{2 \cdot 9.8 \cdot 0.599} = 3.430 \, \text{m/s}
  3. Irod=112β‹…0.950β‹…(1.20)2=0.114 kg\cdotpm2I_\text{rod} = \frac{1}{12} \cdot 0.950 \cdot (1.20)^2 = 0.114 \, \text{kgΒ·m}^2

Substitute into the equation: Ο‰=0.265β‹…(5.242βˆ’3.430)β‹…1.2040.114\omega = \frac{0.265 \cdot (5.242 - 3.430) \cdot \frac{1.20}{4}}{0.114}

Simplify: Ο‰=0.265β‹…1.812β‹…0.300.114=0.144 kg\cdotpm2/s0.114 kg\cdotpm2=1.26 rad/s\omega = \frac{0.265 \cdot 1.812 \cdot 0.30}{0.114} = \frac{0.144 \, \text{kgΒ·m}^2/\text{s}}{0.114 \, \text{kgΒ·m}^2} = 1.26 \, \text{rad/s}


Final Answer:

The angular velocity of the rod is: 1.26 rad/s\boxed{1.26 \, \text{rad/s}}


Let me know if you want any further clarification or additional steps. Here are some related questions to explore:

  1. How does the height the ball reaches after collision affect the angular velocity of the rod?
  2. What would happen if the ball hit the rod at a different point?
  3. How would adding friction at the pivot change the results?
  4. What if the rod had a different shape or moment of inertia?
  5. Can energy conservation be used as an alternate method in this scenario?

Tip: Always check whether a problem involves angular or linear quantities to apply the correct conservation laws

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Math Problem Analysis

Mathematical Concepts

Conservation of Angular Momentum
Energy Conservation
Rotational Dynamics

Formulas

Moment of Inertia (I) = (1/12)ML^2
Linear Velocity: v = sqrt(2gh)
Angular Momentum: L = Iω
Angular Momentum Conservation: L_initial = L_final

Theorems

Conservation of Angular Momentum
Energy Conservation Principles

Suitable Grade Level

Undergraduate Physics